Process for producing thiourea



Dec. 6, 1932. v, HEUSER 1,889,959

PROCESS FOR PRODUCING THIOUREA Filed Feb. 9. 1931 P2 5 N H3 H2OAUTOCLAVE 8 I C 1(OH)2 CALCIUM ACID 6 CYANAMID REACTION VESSEL FILTER a(PO THIOUREA CRYSTALS ATTORNEY Patented Dec. 6, 1932 UNITED STATESPATENT; OFFICE RALPH V. HEUSER, 0F CRANFORD, NEW JERSEY, ASSIGNOR T0AMERICAN CYANAMID COMPANY, OF NEW YORK, N. Y., CORPORATION OF MAINEPROCESS FOR PRODUCING THIOUREA Application filed February 9, 1931.Serial No. 514,386.

This invention relates to the production of thiourea by a reactioninvolving phosphorous pentasulphide and a cyanamid.

In my co-pending application EJerial #510,081, filed January 21, 1931,entitled Method of making thiourea, I have described a highly useful andsuccessful process for the production of thiourea of high purity on alarge scale. By further experimenting I have determined that it ispossible to make an equally pure thiourea bya new method which hassubstantially all of the advantages of the method described in mycopending application.

I have found now that phosphorous pentasulphide can be employedadvantageously in such a process, inasmuch as this reagent not only issuitable to yield hydrogen sulphide by hydrolysis but that it alsosupplies a phosphate anion for precipitating the calcium of the cynamidextract in the form of insoluble tricalcium phosphate. It is thuspossible to avoid the necessity of first converting thecalcium-hydrogen-cyanamid to free cyanamid by means of sulphuric acid orcarbon dioxide. Therefore the reaction in the present instance takesplace between calcium-hydrogencyanamid and a solution of a mixture ofsoluble sulphides and phosphates, derived from the phosphorouspentasulphide. Obviously the aim of separating the calcium as tricalciumphosphate can be realized only in neutral or alkaline solution andadvantage can be taken of the fact that the hydrolysis of phosphorouspentasulphide proceedsmore rapidly in the presence of ammonia or alkalihydroxides than without these bases. When phosphorous pentasulphide, ina pulverized condition, is added to a dilute, cooled solution of ammoniait passes into solution rapidly with formation of an addition product ofvariable composition. On steam-distilling or autoclaving the solution ofthe addition prodnot it is converted into ammonium sulphide anddi-ammonium phosphate.

Essentially quantitative conversions of the sulphur in phosphorouspentasulphide to soluble sulphide-sulphur are realized in this reaction.It is then only necessary to add to the solution of this mixture ofinorganic salts the requisite amount of a filtered calcium cyanamid,extract or a slurry of suspended calcium cyanamid, preferably inconjunction with a small amount of elementary sulphur, in order thatthiourea is formed in solution and calcium precipitated in an insolubleform. The excess of phosphoric acid present in the liquor in form ofammonium phosphate is removed by the addition of a small excess of milkof lime. A

In practicing my invention I proceed for instance, in the followingmanner. 1 kg. of phosphorous pentasulphide of 84 per cent purity isadded to a mixture of 4.06 kg. of aqueous ammonia of specificgravity0.898 at 15 C. and 13 liters of water at a temperature of 15 vto 20 C.The resulting solution is heated in an autoclave at 100 to 120 C. for

approximately two hours. After cooling the resulting solution ofammonium sulphide and diammonium phosphate is then added tothe filteredextract of calcium cyanamid. The

quantity of the latter is computed from the The mixture of the twosolutions is agitated and some kieselguhr is added to facilitate thefilter pressing of the gelatinous precipitate of tri-calcium phosphateas well as the requisite amount of milk of lime for completing theprecipitation. When the reaction is completed as" ascertained by theabsence of cyanamid in the liquor the suspension is'filtered and thefiltrate is heated in a closed vessel for recovering the ammonialiberated in the reaction. The thiourea is separated finally in acrystalline form on cooling the concentrated liquor. The conversionsbased upon cyanamid-nitrogen and phosphorous pentasulphide-sulphur werefound to be above 90 per cent of the theoretical values.

Although the final net reaction appears simple there are a numberv ofintermediate reactions taking place and for clarity, an analysis thereofwill be made. The first stage of the reaction is the addition of ammonia'to phosphorous pentasulphide forming a water and at a high temperatureis converted into di-ammonium phosphate and ammonium sulphide. Thereactions may be represented as follows:

To the resulting product, which is a mix} T It will be noted that thesole reaction products are thiourea and insoluble calcium phos-Lphate,which is filtered off. A large amount of ammonia is regeneratedin this stepand upon evaporation it is easily recovered for re- .use. Inthe accompanying drawing constituting a part hereof, the single figureis a flow sheet showing various steps in the operation of my process.Into the autoclave 1 is placed'a sufiicient amount of water 2, withammonia 3, to form a weak solution ofaqua ammonia and to this is addedphosphorous pentasulphide 4 and a small amount of sulphur 5. The mixtureis heated as mentioned previously and withdrawn after cooling into thereaction vessel 6. After adding thereto the calcium acid cyanamidsolution 7 the mixture is agitated and sluificient milk of lime 8 isintroduced to complete the precipitation of the phosphates at anelevated temperature for. completing ,relatively cheap raw materials andtheoretically it requires only phosphorous pentathe reaction and improvethe filtering qualities of the precipitate. The slurry is then:filteredin the filter press 9 from which the thiourea liquors isconveyed to the evaporator 10 from which the ammonia released isrecovered and re-introduced into the process. The concentrated liquor isrun subsequently .into the crystallizing pan 11 wherein the thioureacrystallizes out'on cooling. If desired the evaporation need not be madeto dryness, but to a suflicient extent so' that upon cooling thioureacrystallizes out.

It will be noted that my process utilizes sulphide, calcium cyanamid anda little lime and sulphur. The thiourea is readily'formed without thenecessityof accurate control or close supervision by skilled chemists.The efficiency of the reaction is very high, it being feasible torecover 90 to 95 per cent of the theoretical quantity of thiourea andthe prod- 1 not is of excellent quality.

- Although I have described my invention, setting forth a specificembodiment thereof and specific conditions of operation and certainsteps of procedure, it will be obvious to the skilled chemist thatvariations in the ingredients andsteps are possible with good results.For example, in place of phosphorous pentasulphide I may use othersulphides of phosphorus, it being merely necessarythat whateversulphides areused they shall react to form the ammonium sulphide andphosphate. In place of ammonia other alkalies may be used, but I use ammonia for convenience because it may be readily separated from thethiourea, and because it is less likely to promote undesirable sidereactions. In place of. calcium;. acid cyanamid, I mayuse calcium orfree cyanamid or other cyanamids such as may come'into phorouspentasulphideand ammonia, adding a cyanamid thereto and recovering thethiourea formed. a v 7 2. A method of making thiourea wh ch comprisescausing'a reaction between phosphorous pentasulphide and ammonla, addingacyanamid solution thereto and recovering the thiourea formed.

3. A method of making thiourea which comprises-causing a reactionbetweenphosphorous pentasulphide and ammonia, adding a'cyanamid capableof forming an insoluble phosphate thereto, and recovering the thioureaformed.

4. A method of making thiourea-which comprises causing a reactionbetween phos phorous pentasulphide and ammonia, adding calciumacidcyanamid thereto, and recoveringthe thiourea formed. g

5. A method of making thiourea whlch comprises causing a reactionbetweenphos-' phorous pentasulphide and ammonia, adding calcium acidcyanamid solution thereto, and

'receovering the thiourea formed.

. 6. A method of making thiourea which comprises causing a reactionbetween phosphorous pentasulphide and ammonia,adding a cyanamid thereto,adding suflicient lime to precipitate the phosphorus as tri-calcium.phosphate and recovering the thiourea formed.

7. A method of making thioureawhich comprises causing a reaction betweenphosphorous pentasulphide and ammonia, adding a cyanamid thereto, addingsufficient lime to precipitate the. phosphorus as 'tri-calciumphosphate, filtering off the phosphate and recovering the thioureaformed.

8. A method of making thiourea which comprises causing a reactionbetween phosphorous pentasulphide and ammonia, adding a cyanamidthereto, adding suflicient lime to precipitate the phosphorus astri-calcium phosphate, filtering oflt' the phosphate, heating thesolution to drive oflf ammonia, and recovering the thiourea formed.

9. A method of making thiourea, which comprises causing a reactionbetween phosphorous pentasulphide and ammonia, adding a cyanamidthereto, adding sufiicient lime to precipitate the phosphorus astri-calcium phosphate. filtering ofit' the phosphate, heating thesolution to drive of! ammonia, evaporating off water and causingthiourea to crystallize.

10. A method of making thiourea which comprises digesting phosphorouspentasulphide and ammonia in the presence of water, adding a cyanamidthereto, and recovering the thiourea formed.

11. A method of making thiourea which comprises digesting under heat andpressure phosphorous pentasulphide and ammonia in the presence of water,adding a cyanamid thereto, and recovering the thiourea formed.

12. A method of making thiourea which comprises digesting under heat andpressure phosphorous pentasulphide and ammonia in the presence of water,adding a cyanamid thereto, allowing the mixture to stand, and recoveringthe thiourea formed.

13. A method of making thiourea which comprises digesting under heat andpressure phosphorous pentasulphide and ammonia in the presence of waterand free sulphur, adding a cyanamid thereto, and recovering the thioureaformed.

In testimony whereof, I have hereunto subscribed my name this fourth dayof February 1931.

RALPH V. HEUSER.

